These experiments are designed to learn more about how the onset of labor is controlled. In particular, the means by which the products of conception influence the onset of labor are being studied. Fetal and placental factors which influence maternal plasma levels and ovarian production of progesterone and estradiol are being sought in the following experiments: 1) we are observing how removal of the fetus in monkeys affects the duration for which the placenta is retained, the maintenance of maternal progesterone and estradiol levels after fetectomy (by radioimmunoassay), and what hormones influence both the levels of those steroids and placental rentention. 2) similar studies in the rabbit, in which fetus, fetal component of the placenta, and maternal component are all readily separable and 3) the effects of dilinoleyl lecithin (DLL) on progesterone production. DLL induces premature sensitization of the pregnant rabbit uterus to oxytocin.